These days, frequency bands allocated to wireless communication are becoming saturated. As measures to deal with this situation, dynamic access concept, which is referred to as “radio opportunistic system (radio-opportunistic)” or “cognitive communication”, is being studied. The principle of this concept is to analyze a frequency spectrum, to avoid a busy occupied frequency bandwidth, to identify and determine an available non-occupied frequency bandwidth, and to change the communication method. However, in order to implement this dynamic frequency access, an ultra-wideband oscillator and a tunable filter are required.
Generally, the reception performance (sensitivity and selectivity) of portable terminals is dependent on a band-pass filter and a mixer. In particular, in order to effectively utilize frequency bandwidths and to implement energy-saving radio telecommunication, a band-pass filter having a high Q factor (Q factor indicates the state of resonance, and as the Q factor is higher, the resonance is more stable) is demanded. As the requirements for a tunable filter, the center frequency can be shifted, and control for increasing or decreasing the passband is necessary. At present, existing oscillation resonators, such as SAW (Surface Acoustic Wave: surface acoustic wave element, and more specifically, a filter element utilizing surface acoustic waves propagating on the surface of a piezoelectric body) and BAW (Bulk Acoustic Wave: a filter element utilizing resonant oscillation of a piezoelectric film itself called bulk acoustic waves) do not satisfy such requirements for a tunable filter. On the other hand, however, a compact tunable band-pass filter which can be fit in a portable terminal has not yet been realized.
As a magnetoresistance effect element, a TMR (Tunnel Magnetoresistive) element is known in which a spacer layer formed of a non-magnetic material is interposed between a pinned magnetization layer and a free magnetization layer. In this TMR element, when a current flows, spin polarized electrons flow and the orientation of magnetization (the orientation of electron spin) of the free magnetization layer changes in accordance with the number of spin polarized electrons stored within the free magnetization layer. In the free magnetization layer disposed in a fixed magnetic field, when the orientation of magnetization is changed, torque acts on electron spin so that the orientation of magnetization will restore to a stable direction constrained by the magnetic field, and oscillation called spin precession occurs when electron spin is oscillated by a specific force.
Recently, the following phenomenon was discovered. When a high-frequency AC current flows into a magnetoresistance effect element, such as a TMR element, strong resonance occurs (spin torque ferromagnetic resonance) when the frequency of the AC current flowing in a free magnetization layer coincides with the number of oscillations of spin precession in which electron spin is returned to the orientation of magnetization (see NPL 1). Moreover, a magnetoresistance effect element is known to exhibit the following function under the following situation. A static magnetic field is externally applied to a magnetoresistance effect element, and the direction of this static magnetic field is tilted within a pinned magnetization layer at a certain angle with respect to the direction of the magnetization of the pinned magnetization layer. In this state, when an RF current (RF current having a frequency which coincides with the number of oscillations of spin precession (resonant frequency)) is input into the magnetoresistance effect element, the magnetoresistance effect element is known to exhibit a function of generating a DC voltage proportional to the square of the amplitude of the input RF current across the magnetoresistance effect element. That is, the magnetoresistance effect element exhibits a square law detecting function (spin torque diode effect). It is also known that this square law detection output of the magnetoresistance effect element exceeds a square law detection output of a semiconductor pn junction diode under certain conditions (see NPL 2).
The applicant of this application has focused on the square law detecting function of a magnetoresistance effect element and studied the application of such a magnetoresistance effect element to a mixer which is operable with low local power, and has already proposed such a mixer (see PTL 1). A mixer using a magnetoresistance effect element includes a magnetic-field applying unit that applies a magnetic field to the above-described free magnetization layer, and when a first high frequency signal S1 and a second high frequency signal S2 for a local signal are input, the mixer generates a multiplication signal S4 due to a magnetoresistance effect. However, the multiplication signal S4 considerably attenuates if it directly passes through a 50-Ω matching circuit. Accordingly, the applicant of this application has proposed that an impedance circuit (a filter, a capacitor, or an active element) is inserted between an input transmission line through which the first high frequency signal S1 and the second high frequency signal S2 are transmitted and the above-described magnetoresistance effect element so that the impedance for the multiplication signal S4 will become higher than the impedance for the first high frequency signal S1 and the second high frequency signal S2 (see PTL 2).
Although the characteristics of a mixer using the above-described magnetoresistance effect element are known, a high frequency device that can apply such characteristics to an industrial use is still unknown. Accordingly, the discovery of the application of such characteristics to an industrial use has been expected. The applicant of this application has found that, by using the square law detecting function of a magnetoresistance effect element, a multiplication signal output increases and decreases in accordance with resonance characteristics and the frequency selective function is exhibited. However, a high Q factor has not been obtained, and the frequency selection range is too wide, and thus, the applicant of this application has not yet found the application of the above-described characteristics to an industrial use.